Pseudomigraine with temporary neurologic symptoms and lymphocytic pleocytosis is a self-limited syndrome of unknown origin characterized by headache accompanied by transient neurologic symptoms and cerebrospinal fluid lymphocytosis. Patients with this condition are between 15 and 40 years of age. The syndrome is more frequent in men. The clinical picture encompasses one to 12 episodes of changing variable neurologic deficits accompanied by moderate to severe headache and occasional fever. These headaches are described as predominantly throbbing and bilateral with a variable duration (mean, 19 hours). The average duration of the transient neurologic deficit is 5 hours. Sensory (78% episodes), aphasic (66%), and motor (56%) disturbances are the most common. Migraine-like visual symptoms are relatively rare (18% episodes). Patients are asymptomatic between episodes and after the symptomatic period (duration > 3 months). Lymphocytic pleocytosis (10 to 760 cells mm(3)) and increased cerebrospinal fluid protein are found with negative bacteriologic, viral, fungal, and immunologic studies. Brain computed tomography and magnetic resonance imaging are normal, but an electroencephalogram frequently shows focal slowing over the symptomatic brain area. Single photon emission computed tomography reveals transient focal areas of decreased uptake consistent with the clinical symptoms. It is possible that pseudomigraine with temporary neurologic symptoms and lymphocytic pleocytosis could result from an activation of the immune system secondary to a recent viral infection, which would produce antibodies against neuronal or vascular antigens. This autoimmune attack may induce an aseptic leptomeningeal vasculitis, accounting for the headache and the transient symptoms likely through a spreading depression-like mechanism.